The present invention is directed, in general, to magnetic devices, and, more specifically, to a magnetic device employing an isolation barrier and a method of manufacture therefor.
Electronic manufacturers are constantly striving to make electronic components even smaller, especially in the field of magnetic devices. Miniature magnetic devices are used in a wide variety of electronic equipment such as telephones, televisions, computers, etc. With overall dimensions of the devices becoming smaller, the design of the magnetic devices presents unique challenges. The structure of the miniature devices must accommodate special features that are necessary to the manufacturability and electrical performance thereof.
A magnetic device is a device that uses magnetic material arranged in a defined structure for shaping and directing magnetic fields in a predetermined manner to achieve a desired electrical performance. The magnetic fields in turn act as the medium for storing, transferring and releasing electromagnetic energy. As a specific example of magnetic devices, transformers are composed of two or more windings wound about a bobbin with a magnetic core inserted through the bobbin. The bobbin may be made of virtually any suitable dielectric material. The insulated windings are wound about the bobbin in patterns to achieve specific electrical characteristics. The number of windings and the number of turns per winding is dictated by the function of the transformer in the intended circuit.
The bobbin may be manufactured separate from or integral with a base that provides the physical support for the bobbin. The transformer is electrically connected to the circuit by contacts extending from the base. A core of magnetic permeability, often ferrite, is inserted into the bobbin to shape the magnetic field. The core is often made in two pieces with an xe2x80x9cExe2x80x9d shaped cross section. The central poles of the E-shaped core halves are inserted into opposite ends of the bobbin and the poles at the center of the bobbin. The complete transformer assembly is held together by various physical means such as an adhesive or spring clip.
Transformers work on the general principle that a change in current flowing through the primary winding, which is isolated from the secondary winding, creates a magnetic flux which causes a change in the current to flow in the secondary winding. The ratio of primary-to-secondary current is established by the number of windings in the secondary coil related to the number of windings in the primary coil. This, in turn, creates a voltage which is the product of the number of turns multiplied by the change in flux. This product is also proportional to a change in current multiplied by the inductance.
As the electronic devices employing magnetic devices continue to be made smaller, it is necessary to design a more compact and lower profile magnetic device (e.g., transformer). The limitation of xe2x80x9ccreepage,xe2x80x9d however, can adversely affect the design and operation of the magnetic device. Creepage is generally defined as the transference of electrical current from one winding in a transformer to another winding in the same transformer by way of a conductive path forming a temporary bridge along a surface of a dielectric material separating the windings. The leakage current generally occurs as a result of ionization of air and insufficient creepage distance. Additionally, a minimum creepage distance is often required to comply with safety standards.
For example, a transformer may have a primary winding wound about a bobbin with a primary lead extending therefrom coupled to an input terminal of the transformer. In addition, the transformer may have a secondary winding wound concentrically about the same bobbin and around the primary winding. Although the primary winding and the secondary winding are separated by an insulator, the lead coming from the primary winding and terminating at the input terminal passes very close to the secondary winding. Because the primary lead passes very close to the secondary winding, there is the likelihood of creepage between the primary and secondary windings (e.g., at the point where the lead of the primary winding and the secondary winding are in the closest proximity to each other) thereby resulting in a potential short-circuit in the transformer. The distance between the point on the primary lead and the point on the secondary winding along the surface of which the creepage occurs is commonly known to those skilled in the art as the creepage distance.
As alluded to above, to help prevent creepage in magnetic devices, standards have been instituted defining minimum insulation permitted in a transformer. The standards are promulgated by administrative bodies such as the International Electrotechnical Commission (IEC) to, among other things, increase the safety of devices employing components such as magnetic devices (see, for instance, IEC Standard 60950, third edition, 1999). Included in these standards are the minimum creepage distances depending on the specifications of the transformer and the circuit into which the transformer is to be employed. The standards are becoming more universally accepted. Therefore, minimum creepage distance in magnetic devices is becoming a more important factor in the design of such devices.
In the prior art, two commonly employed methods are employed to assure a minimum creepage distance. The first method is to manufacture two separate bobbins, each one with a winding wound thereabout and leads affixed to the respective terminals. One bobbin is then placed inside of the other so that the leads of one winding and the other winding are isolated from each other. Although effective for ensuring a minimum creepage distance, the use of two bobbins has several disadvantages. First, the two-bobbin method requires the manufacture of two bobbins rather than only one, thus resulting in increased parts and manufacturing costs. Second, the use of two bobbins is better suited for larger transformers because of the difficulty associated with manufacturing miniature bobbins that fit together, one inside the other. Third, the use of one bobbin inside of another commonly leads to a large leakage inductance as a result of the space between the windings. Thus, the two bobbin approach is not the design of choice.
The second and more commonly employed method to assure a minimum creepage distance is the use of sleeves placed on the wire leads of the inner winding. However, like the two bobbin approach discussed above, the use of wire sleeves also has major disadvantages. Although the use of wire sleeves is employable in the manufacture of small transformers, placing the wire sleeves on each of the leads of the inner winding is a labor-intensive process that must be completed by hand. As a result, the costs of manufacturing small transformers having wire sleeves on the leads of the inner windings is high. In addition, although placing sleeves on the large leads of large transformers may appear at first glance to be a trivial task, placing sleeves on extremely small leads of miniature transformers becomes a tedious and time-consuming chore. As a result, labor costs, and thus the overall costs of manufacturing, are again elevated.
Accordingly, what is needed in the art is a magnetic device, and related method, that maintains a predetermined creepage distance therein, but overcomes the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a magnetic device and a method of manufacture therefor. In one embodiment, the magnetic device includes: (1) a bobbin having a winding guide (or core tube) and molded-in margins proximate opposing inside flanges of the bobbin, each of the opposing inside flanges having at least one notch formed in an inside face of each of said opposing inside flanges; (2) an inner winding wound about the winding guide and between the molded-in margins; (3) an outer winding wound about the inner winding and the winding guide and between the flanges; and (4) an insulating plate, provided in the at least one notch and interposed between the inner and outer windings, a thickness of the insulating plate providing a predetermined creepage distance between at least one lead of the inner winding and the outer winding.
The present invention, in one aspect, introduces the broad concept of a magnetic device employing a bobbin having flanges adapted to receive an isolation barrier. The isolation barrier is interposed between inner and outer windings of the magnetic device to allow a creepage distance between the inner and outer windings to be above a predetermined amount. The magnetic device may thus meet insulation requirements promulgated by various standards bodies, such as the International Electrotechnical Commission (IEC).
In one embodiment of the present invention, the magnetic device further includes a magnetic core proximate the inner and outer windings. The magnetic core may thus impart a desired magnetic property to the inner and outer windings. In a related embodiment, the magnetic device further includes a spring clip that retains the magnetic core on the bobbin. The magnetic core may, in an advantageous embodiment, include first and second magnetic core portions. The spring clip may thus secure the first and second magnetic core portions to the bobbin. Those skilled in the pertinent art realize, of course, that the spring clip is not necessary to practice the present invention.
In one embodiment of the present invention, the isolation barrier includes an insulating plate. The insulating plate may be a molded plastic plate, a polyamide plate, or a nomex plate. Of course, the use of other materials for the insulating plate is well within the broad scope of the present invention. In a related embodiment, the isolating barrier further includes a layer of insulating tape placed over the insulating plate. The insulating tape may thus secure the insulating plate in place within the magnetic device. Of course, placing the insulating tape over the insulating plate is not necessary to practice the present invention.
In one embodiment of the present invention, the isolation barrier provides the creepage distance between a lead of the inner winding and a body of the outer winding. The magnetic device may thus avoid the use of sleeving to compensate for the lack of creepage distance between the lead of the inner winding and the body of the outer winding.
In one embodiment of the present invention, the predetermined amount is determined by a safety standard. The safety standards may be promulgated by any of various standards bodies, such as Underwriters Laboratories, Inc. (UL) and the IEC. Those skilled in the pertinent art are familiar with these and other relevant standards bodies.
In one embodiment of the present invention, the molded-in margins are slotted to allow at least one lead of the inner winding to terminate on a winding terminal of the magnetic device. In a related embodiment, the flanges are slotted to allow at least one lead of the outer winding to terminate on a winding terminal of the magnetic device. The leads of the inner and outer windings may thus be directly connected to the respective winding terminals while maintaining the predetermined creepage distance.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the pertinent art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the pertinent art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the pertinent art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.